During his years on “Saturday Night Live,” Jimmy Fallon was known as a likable actor with several spot-on impressions, including Adam Sandler and Jerry Seinfeld. He also developed some mildly amusing characters, such as Webcam stoner Jarret and computer geek Nick Burns. And he had the good fortune to have Tina Fey by his side on “Weekend Update.”

His downside, however, was an ever-increasing inability to curb his midshow laughter. Fellow “SNL” alumnus Tracy Morgan even told Penthouse recently that Fallon’s castmates became fed up with his crackups, and that Morgan had warned Fallon to curb the “laughing and all that dumb [bleep]” he used to do.

“That’s taking all the attention off of everyone else and putting it on you,” said Morgan, describing a terrible trait for a comic actor — and an even worse one for a talk-show host.

So when Jimmy Fallon was announced as Conan O’Brien’s “Late Night” replacement, a writer for Entertainment Weekly echoed the thoughts of many by writing, “I don’t know that I want to watch him giggle his way through interviews night after night.”

Whatever Fallon’s failings, the early jabs also perpetuated a ritual of harsh pre-premiere scrutiny for late-night hosts, as newbies from Conan O’Brien to Jimmy Kimmel began their gigs with the kind of train-wreck expectations that usually accompany of a Britney Spears reality show. Remember that O’Brien was so poorly regarded at first that NBC would only renew his contract 13 weeks at a time.

Given these unique pressures, then, it’s not surprising that several members of late night’s exclusive fraternity called Fallon with advice and support on his first day.

While Stewart was comically warning him not to overthink the job, Stephen Colbert offered a more serious indication of what lay ahead.

“Colbert gave me a piece of advice that [Johnny] Carson gave to Conan, and that Conan told Colbert,” Fallon says. “It was, ‘With this job, you’ll use everything you’ve ever known.’ And that is great advice. I checked that off my list the first week.”

While Fallon’s first show was a ratings triumph despite a stiff interview with Robert De Niro, the reviews were mixed, with many taking note of Fallon’s frayed nerves.

“I think the critics were fair,” he says. “I got good reviews and bad reviews, and even [with] the bad reviews, I go, ‘OK. I can agree with that.’ I don’t know how to host a talk show. I don’t know what I’m doing. I gotta learn how to do it.”

To his credit, Fallon took some of the critiques to heart.

“Newsweek, I think, said I was sweaty, and they were right. My face was so sweaty, wet and greasy that makeup couldn’t even help me. So we’ve lowered the temperature in the studio. I also stopped drinking as many Red Bulls, and I’ve calmed down a bit.”

Also, knowing the chasm that frequently lies between critical acclaim and mass popularity in comedy (i.e., Larry the Cable Guy), certain criticisms were ignored, such as those that slammed “Lick It For Ten,” a bit where audience members are invited to lick random items for $10.

“I’m here for a reason. I think I know what’s funny or what’s good,” says Fallon. ” ‘Lick It for Ten,’ none of the critics enjoyed, but the fans loved it, and we kept doing it. After the third or fourth show, I was walking down the street, and some kid turns to me and says, ‘Hey, Jimmy Fallon. Lick it for 10!’ I go, ‘Oh, God. What have I done?’ “

After working in the LA improv group The Groundlings, spending seven years on “SNL” and starring in a handful of big-budget films, Fallon would seem to be an old hand before the camer. But in his first month and a half on the air, he’s had to cope with the “grind” of running a late-night talk show, including the challenges involved with interviewing celebrities.

“Sometimes people backstage will say, ‘I got this great story about karate and it’s really funny,’ and you go, ‘OK, great.’ So onstage, you go, ‘You just recently took karate,’ and they go, ‘No, not really.’ And we’ve got six minutes left. When it doesn’t work, it creeps by so slow.”

The trick to being a successful late-night host, as Carson proved, is making it seem like a breeze, despite the challenges. While he acknowledges that his training is just beginning, Fallon says the most important thing he’s learned is, “It’s not all about me. It’s about my guests, and about making them feel good and look good.”

To that end, he engages them in activities outside of late night’s usual promotional babble, such as: playing foosball with Clive Owen, competing against Cameron Diaz in a dance-off and enlisting Drew Barrymore in “Lick It for Ten,” with the actress eagerly affixing her tongue to a bowling ball.

Equally important for Fallon is engaging his fans, as he recently did when one young lady got to sing karaoke with Jon Bon Jovi.

Fallon will reach out off the show as well, such as this Saturday, when he co-hosts the EIF Revlon Run/Walk for Women (RevlonRunWalk.com), where 40,000 people will run and walk from Times Square to Central Park’s East Meadow to benefit women’s cancer research.

If Fallon has grown since his “SNL” days, it’s in the realization that a successful host allows others to shine.

“Jack Benny once said, ‘I’m not funny. [My sidekick] Rochester is funny. But people see the show, and they come into work and say, “See how funny Jack Benny was last night,” ‘ ” says Fallon. “You don’t have to be the funny guy all the time. You can let other people be funny and score, and that’s great. I want people to come on my show and score.”